Bob,
Do you know the model of the coin mech?
I do recall someone posting this solution once before; however, adding a free button may be easier and less aggravation... Depending on where the button is to be mounted...
I briefly looked for the post to no avail
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I'm not sure if this will work because these vending mechs can get a little complicated, but try tie-strapping the solenoid arm back and keeping the latch from locking so you can vend a soda and possibly continue to vend free bottles. The latch is spring loaded so it may attempt to reset after each vend. Removing the latch and spring may be the best bet...
If you need a diagram, let me know... I have a diagram from a V90 which should be similar